Building block



FIG. 1 is a perspective view of a building block;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a left side elevational view thereof;

FIG. 4 is a top plan view thereof;

FIG. 5 is a back elevational view thereof;

FIG. 6 is a right side elevational view thereof; and

FIG. 7 is a bottom plan view thereof. 

The ornamental design for a building block, as shown. 